It has become increasingly important to determine the location of a mobile telephone or other mobile device capable of radio communication. One method of assessing geolocation of a mobile device is using the mobile device in conjunction with a geolocation system. Such geolocation systems include, for example, the Navstar Global Positioning System (i.e., GPS). GPS is a radio positioning system which provides its subscribers with highly accurate position, velocity, and time (PVT) information. The GPS includes a constellation of GPS satellites in non-geosynchronous 12 hour orbits around the earth.
FIG. 1 is a schematic representation of constellation 100 of GPS satellites 101. The GPS satellites 101 travel in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of on orbit spare satellites. Each orbital plane has an inclination of 55 degrees relative to the equator. In addition, each orbital plane has an altitude of approximately 20,200 km (10,900 miles). The time required to travel the entire orbit is about 12 hours. Thus, at any given location on the surface of the earth at least five GPS satellites are visible at any given time.
GPS position determination is made based on the time of arrival (TOA) of various satellite signals. Each of the orbiting GPS satellites 101 broadcasts spread spectrum microwave signals encoded with positioning data and satellite ephemeris information. The signals are broadcast on two frequencies: 1575.42 MHz (L1) and 1227.60 MHz (L2). The L1 frequency carries the navigation data as well as the standard positioning code, while the L2 frequency carries only the P code and is used for precision positioning code for military applications. The signals are modulated using bi-phase shift keying techniques. The signals are broadcast at precisely known times and at precisely known intervals and each signal is encoded with its precise transmission time.
A GPS subscriber receives the signals with a GPS receiver configured to time the signals and to demodulate the satellite orbital data (ephemeris information) contained in each signal. Using the ephemeris information, the receiver can determine the time difference between transmission of the signal by the satellite and its reception by the receiver. Multiplying the time difference by the speed of light provides a pseudo range measurement of that satellite. Assuming that the receiver's clock is perfectly synchronized with the satellite clocks, this information could yield actual range measurement for each satellite. However, the clock drift at the receiver can cause it to differ by time offset value thereby making the accuracy of a pseudo range measurement questionable.
Because the time offset is common to the pseudo range measurement of all the satellites, the pseudo ranges of four or more satellites enables the GPS receiver to determine the time offset and its location in three dimensions. Thus, a receiver is able to determine his PVT with great accuracy, and use this information to navigate safely and accurately from point to point, among other uses.
However, the signal received from each of the visible satellites does not necessarily result in an accurate position estimation. The satellite pseudo-ranges measured and reported by GPS receivers are sometimes inaccurate. For example, this may occur due to “multipath” effects, i.e., where the GPS signal has not taken a direct path to the receiver but instead has, for example, bounced off a building.
Accordingly, there is a need for a method and apparatus for geolocation determination that would overcome this problem.